Irish writer and director, Marion Wyatt, is turning her parents' love letters into a play after she discovered a tragic family secret in their shared correspondence.

Marion discovered a collection of cards and letters after the death of her mother, written by her parents between 1952 and 1975.

"There were anniversary cards, little postcards from when he was away at sea, a little autograph book and a diary where she, as a 16-year-old, wrote some really innocent but intimate things about him," Marion explained to Matt Cooper on Today FM.

Even when Marion's father died in 1975 her mother continued to write to him, especially if there was "a moment of extreme happiness or change in the family".

"I wasn't aware that she was doing it. We knew that she was fond of writing little notes because she was one of the generation who would write cards to everyone. She was always sending little thank you notes and reared us to do the same."

The unsent correspondence was hidden in the family home and Marion found little notes in the most unusual of places, some thoughts were even written down on the back of receipts.

Tina and Finbar Vaughan. When We Were Young.

"She might have been watching something on TV or heard a song on the radio and she would have written down the lyric and then that almost was like her brainstorming with herself. The lyric of the song would set her off on some kind of journey, remembering something else,"said Marion of her mother's notes.

The director said she did have some concerns about reading her mother's private and innermost thoughts but the discovery helped her to understand her parents better and, in turn, herself.

"I shed many a tear, of course I did because I discovered something about her, her early years and about their early marriage, before I was born and reading that, my imagination took over.

"I thought 'Oh that's why she had blue days' or 'That's why she had very sad moments' and then that for me, set me off on a journey to investigate further."

The journey led Marion to discover the big family secret and that is the kickstarter of the subject of her play, When We Were Young.

"They had a daughter before I was born and that daughter died at birth but they never told anyone and they never spoke about her in their entire married life.

Letters between Marion Wyatt's mother and father - the subject of her new play When We Were Young.

"She never spoke of it, he never spoke of it and they never spoke of it together. They never shared that secret," said Marion of the tragic family.

"When my father was dying, he was on morphine in the last few days of his life, he thought I was his mother. Now on reflection when I think back, he was saying 'Put daffodils on her grave' - sure I thought he was talking about his own mother's grave but in actual fact he was talking about my sister," said the writer.

"[The discovery] helped me on a personal journey to put at rest some bad memories from my childhood because it's helped me to understand why they might not have been as kind as they should have been to us as children."

Marion described how her father often displayed cruelty in his personality which she believes was "born from a pain that hadn't been expressed".

"I suffered some beatings which were calculated, not as a result of alcoholism or anything like that, I think he just wanted me to be perfect and I think he had this ambition for us all to be perfect... to be high achievers.

"There were absolutely wonderful things about him as well as he raised his daughters to be as good as the boys and vice versa but there was just this part of his personality that I needed to understand because I became sick as a result of it," explained Marion.

"The play isn't about abuse, it's about discovering why he was the way he was and why my mother had sad days.

"When I found the letters and notes it helped me to see that, yes they were very young, they loved each other deeply, they fell head over heels in love... he gave up his ambition of seeing the entire world for her and they committed to each other and reared us the very best they could."

When We Were Young is running between April 6 and 12 in The Everyman Theatre in Cork.